Assessment of the plug flow and dead volume ratios in a sub-surface horizontal-flow packed-bed reactor as a representative model of a sub-surface horizontal constructed wetland

Abstract

The aim of this study was to assess the influence of design parameters on the plug flow and dead volume ratios in a sub-surface horizontal-flow packed-bed reactor (HPBR), as a representative model of a sub-surface horizontal-flow constructed wetland (SSHFCW), to provide a further understanding of SSHFCW, and to assist in the design and application of these systems on a larger scale. Design parameters included the aspect ratio, the size of the porous media and the loading rate of the HPBR. Integral parameters, such as plug flow, dead volume and short-circuiting ratios, were determined using tracer tests and the Wolf–Resnick empirical flow model. The experiment was conducted in three packed-bed reactors with volumes of 192L and different length-to-width aspect ratios (1:1, 1.5:1 and 3:1). Each packed-bed reactor received three different controlled influent hydraulic loading rates (0.3L/min, 0.6L/min, and 1.5L/min), and the size of the porous media was varied in the individual beds, using three nominal diameters (4.7mm, 9.2mm and 12.7mm). Results showed that the aspect ratio had the greatest effect on the hydraulic behaviour of the system. An increase in the aspect ratio resulted in a experimental retention time that was closer to the theoretical retention time, and exhibited higher plug flow and lower dead volume ratios compared to an ideal plug flow system operated under similar conditions. An increase in loading rate negatively affected the plug flow portion in the system by increasing dispersion. The retention time increased, making it more similar to ideal plug flow and reducing the dead volume ratio. Finally, it was demonstrated that a decrease in the nominal diameter increased dispersion and reduced the plug flow ratio; however, the retention times obtained were closer to those of the ideal plug flow system. It was concluded that a constructed sub-surface flow wetland design should incorporate a combination of higher aspect ratios, higher loading rate and finer porous media to induce a hydraulic behaviour closer to an ideal plug flow system.